1. Field of the Invention
The present invention relates to a magnetic encoder forming a part of a rotation detecting device for detecting the rotational speed of a rotary element such as, for example a wheel, and a wheel bearing assembly utilizing such magnetic encoder.
2. Description of the Prior Art
A magnetic encoder utilizing a magnetic rubber has hitherto been largely employed in a wheel bearing assembly for supporting a vehicle wheel. This type of magnetic encoder is of a structure in which an elastic member mixed with a powder of magnetic material is integrated with a core metal in a ring-shaped configuration with different magnetic polarities formed alternately in a direction circumferential of the ring-shaped elastic member. This magnetic encoder is, when it is in use, positioned in front of a magnetic sensor with an air gap intervening therebetween, to thereby complete the rotation detecting device for detecting the rotational speed of, for example, a vehicle wheel.
In recent years, the need has arisen to increase the size of the air gap between the magnetic encoder and the magnetic sensor so that the manufacturing cost of the rotation detecting device for the wheel bearing assembly of the kind referred to above can be suppressed by mitigating the stringent design tolerance in dimensional variation that occurs during assemblage of component parts of the rotational detecting device. Since the magnetic encoder is, when it is an axial type, susceptible to deflection in an axial direction during rotation thereof, the air gap is required to be set to a large value to a certain extent so that change in spacing between the magnetic encoder and the magnetic sensor resulting from the deflection can be accommodated in the tolerance requirement for assemblage of the component parts of the rotation detecting device. While the air gap is generally designed to be of a size within the range of 0.5 to 2.0 mm, requirement to suppress the manufacturing cost necessitates the air gap to be further increased to a value higher than 0.5 to 2.0 mm.
In order to increase the size of the air gap, it is necessary to increase a magnetic force produced by the magnetic encoder. In the conventional quality control, the magnetic force developed by the magnetic encoder has been controlled by measuring the magnetic flux density with the magnetic sensor brought close to the magnetic encoder. However, measurement with the use of the magnetic sensor involves some problems in that it takes a relatively long time to complete the measurement and in that measurements tend to vary depending on the site of measurement (measuring environment). To suppress the manufacturing cost, the need has arisen to ease the quality control as well.